Here are some useful tips for poster artists from the blog Scamp written by a creative/creative director at BBH in London. He tells use that someone once said: "A poster should have no more than seven words on it."
That person made a schoolboy error.
You see, a poster should have NO words on it.” He goes on to explain why.
Scamp writes that no one is looking at a poster unless you make them.
To catch their an eye, you need an image. I you use no words you are forced to use a strong self-explanatory image that will catch the eye and stick with their mind. This makes sense as people often ignore the words anyway and the words are not what draws you to the poster unless the poster uses very provocative words. Then Scamp writes, you should not use an image as it will compete with the words. Thanks to the IF team for pointing this out. There are 30 other tips on Scamp’s blog so take a look.